Large scale, monodispersed octahedral BiVO4 microcrystals, photostability and water oxidation thereof

The present disclosure relates to a method of making bismuth vanadate particles, and a method of using bismuth vanadate particles as part of a photoelectrode for water splitting.

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.

Facet-controlling has become an attractive method to enhance the catalytic behavior of semiconducting photocatalysts through modification of their morphology and crystallinity. Photochemical processes including light absorption and charge separation could be improved using photocatalysts having a specifically designed morphology and surface atomic structures [Y. Zhao, R. Li, L. Mu, C. Li, Significance of Crystal Morphology Controlling in Semiconductor-Based Photocatalysis: A Case Study on BiVOPhotocatalyst, Cryst. Growth Des. 17 (2017) 2923-2928, incorporated herein by reference in its entirety]. For instance, morphological and facet tuning of anatase TiO{001} facets created highly photoactive species. Because of the large surface area of {001} facets, the exposed facets of anatase TiOgrown as a single crystal generated an efficient photoresponse. Controlling the morphology and crystallinity of Platonian-shaped (tetrahedra, hexahedra, octahedra, etc.) metal oxides and metal vanadates is a challenge to research efforts because less is known about their crystallization than non-Platonian shaped metal oxides [X. Wang, D. Liao, H. Yu, J. Yu, Highly efficient BiVOsingle-crystal photocatalyst with selective AgO—Ag modification: Orientation transport, rapid interfacial transfer and catalytic reaction, Dalt. Trans. 47 (2018) 6370-6377, incorporated herein by reference in its entirety].

Metal vanadates are an emerging class of photocatalytic materials for photoelectrochemical applications. Among metal vanadates, BiVO(BV) has attracted attention for satisfactory optoelectrical characteristics (e.g., a bandgap of ˜2.40 eV), along with ionic conductivity, ferroelasticity, acousto-optical, and photochromic properties. Further, BV possesses high chemical stability and low toxicity [K. Wenderich, G. Mul, Methods, Mechanism, and Applications of Photodeposition in Photocatalysis: A Review, Chem. Rev. 116 (2016) 14587-14619; S. Wang, G. Liu, L. Wang, Crystal Facet Engineering of Photoelectrodes for Photoelectrochemical Water Splitting, Chem. Rev. 119 (2019) 5192-5247; and J. K. Cooper, S. Gul, F. M. Toma, L. Chen, Y. S. Liu, J. Guo, J. W. Ager, J. Yano, I. D. Sharp, Indirect bandgap and optical properties of monoclinic bismuth vanadate, J. Phys. Chem. C. 119 (2015) 2969-2974, each incorporated herein by reference in their entirety]. Under visible light irradiation, BV materials can be used in photoelectrochemical (PEC) applications as photoanodes. Because of its suitable band positions, chemical stability, sensitivity to visible light, and low cost, BV may be an alternative photocatalyst to TiOfor solar energy conversion and environmental protection applications [G.-L. Li, First-principles investigation of the surface properties of fergusonite-type monoclinic BiVOphotocatalyst, RSC Adv. 7 (2017) 9130-9140; and H. Gong, N. Freudenberg, M. Nie, R. Van De Krol, K. Ellmer, BiVOphotoanodes for water splitting with high injection efficiency, deposited by reactive magnetron co-sputtering, AIP Adv. 6 (2016), each incorporated herein by reference in their entirety].

In view of the forgoing, one objective of the present disclosure is to provide a method of preparing bismuth vanadate particles. The bismuth vanadate particles may be present on a conducting substrate to form a photoelectrode used, for example, in a photoelectrochemical cell for water splitting.

According to a first aspect, the present disclosure relates to a method of preparing bismuth vanadate particles that have an octahedral shape and comprise crystalline BiVOhaving a tetragonal crystal symmetry. The method involves the steps of (i) mixing a bismuth(III) salt and an acidic aqueous solution to form a bismuth mixture, (ii) mixing a metavanadate salt and a basic aqueous solution to form a vanadate mixture, (iii) mixing the vanadate mixture and the bismuth mixture to form a reaction mixture that is simultaneously subjected to ultrasonication, and (iv) hydrothermally treating the reaction mixture, thereby forming the bismuth vanadate particles.

In one embodiment, the bismuth vanadate particles have an average particle size in a range of 0.8-6 μm.

In one embodiment, the reaction mixture is subjected to ultrasonication at a frequency in a range of 17.5-22.5 kHz.

In one embodiment, the reaction mixture is subjected to ultrasonication at an amplitude in a range of 8-35%.

In a further embodiment, the reaction mixture is subjected to ultrasonication at an amplitude in a range of 18-22%, and the bismuth vanadate particles consist essentially of crystalline BiVOhaving a tetragonal crystal symmetry.

In one embodiment, the reaction mixture is hydrothermally treated at a temperature of 100-250° C.

In one embodiment, the bismuth(III) salt is bismuth(III) nitrate.

In one embodiment, the metavanadate salt is ammonium metavanadate.

In one embodiment, the acidic aqueous solution comprises nitric acid, and the basic aqueous solution comprises sodium hydroxide.

In one embodiment, the bismuth mixture further comprises an ionic surfactant.

In a further embodiment, the ionic surfactant is sodium dodecylbenzene sulfonate.

According to a second aspect, the present disclosure relates to a BiVOphotoelectrode, which includes (i) a conducting substrate, and (ii) bismuth vanadate particles having an octahedral shape and comprising crystalline BiVOhaving a tetragonal crystal symmetry, present on a surface of the conducting substrate.

In one embodiment, the bismuth vanadate particles have an average particle size in a range of 0.8-6 μm.

In one embodiment, the conducting substrate is fluorine-doped tin oxide.

According to a third aspect, the present disclosure relates to a photoelectrochemical cell including the BiVOphotoelectrode of the second aspect, a counter electrode, and an electrolyte solution containing water and an inorganic salt in contact with both the BiVOphotoelectrode and the counter electrode.

In one embodiment, the electrolyte solution has an inorganic salt concentration of 0.05-1 M.

In one embodiment, the photoelectrochemical cell further includes a reference electrode.

In one embodiment, the BiVOphotoelectrode has a photo-current density in a range of 0.15-1.2 mA/cmwhen the photoelectrochemical cell is subjected to a bias potential of 0.5-1.3 V vs RHE under visible light irradiation.

In one embodiment, the BiVOphotoelectrode has a photo-to-current conversion efficiency (IPCE) in a range of 2-25% when the photoelectrochemical cell is subjected to a bias potential of 1.23 V vs RHE under visible light irradiation, and an applied bias photo-to-current efficiency (ABPE) in a range of 0.02-0.3% when the photoelectrochemical cell is subjected to a bias potential of 0.7-0.85 V vs RHE under visible light irradiation.

According to a fourth aspect, the present disclosure relates to a method of splitting water into Hand O. The method involves subjecting the photoelectrochemical cell of the third aspect to a bias potential of 0.5-2.0 V, and concurrently irradiating the photoelectrochemical cell with visible light, thereby forming Hand O.

The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.

Embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the disclosure are shown.

The present disclosure may be better understood with reference to the following definitions. As used herein, the words “a” and “an” and the like carry the meaning of “one or more.” Within the description of this disclosure, where a numerical limit or range is stated, the endpoints are included unless stated otherwise. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Although the open-ended term “comprising,” as a synonym of terms such as including, containing, or having, is use herein to describe and claim the present composition and methods, the composition and/or methods may alternatively be described using more limiting terms, such as “consisting of” or “consisting essentially of” the recited components/steps. For example, a composition which consists essentially of the recited components may contain other components which do not adversely affect the photoelectrochemical properties of the composition.

As used herein, the words “about” or “substantially similar” may be used when describing magnitude and/or position to indicate that the value and/or position described is within a reasonable range of values and/or positions. For example, a numeric value may have a value that is +/−0.1% of the stated value (or range of values), +/−1% of the stated value (or range of values), +/−2% of the stated value (or range of values), +/−5% of the stated value (or range of values), +/−10% of the stated value (or range of values), or +/−15% of the stated value (or range of values).

The present disclosure includes all hydration states of a given salt or formula, unless otherwise noted. For example, bismuth(III) nitrate includes anhydrous Bi(NO), pentahydrate Bi(NO)·5HO, and any other hydrated forms or mixtures.

The present disclosure further includes all isotopes of atoms occurring in the present compounds. Isotopes include those atoms having the same atomic number but different mass numbers. By way of general example, and without limitation, isotopes of hydrogen include deuterium and tritium, and isotopes of oxygen includeO,O andO. Isotopically labeled compounds of the disclosure can generally be prepared by conventional techniques known to those skilled in the art or by processes and methods analogous to those described herein, using an appropriate isotopically labeled reagent in place of the non-labeled reagent otherwise employed.

Recently, BV materials have been studied for photoelectrochemical (PEC) water splitting applications [I. Khan, S. Ali, M. Mansha, A. Qurashi, Sonochemical assisted hydrothermal synthesis of pseudo-flower shaped Bismuth vanadate (BiVO) and their solar-driven water splitting application, Ultrason. Sonochem. 36 (2017) 386-392; N. Iqbal, I. Khan, Z. H. A. Yamani, A. Qurashi, A facile one-step strategy for in-situ fabrication of WO—BiVOnanoarrays for solar-driven photoelectrochemical water splitting applications, Sol. Energy. 144 (2017) 604-611; S. Ali, I. Khan, S. A. Khan, M. Sohail, Z. H. Yamani, M. A. Morsy, M. Qamaruddin, Plasmon aided (BiVO)—(TiO), ternary nanocomposites for efficient solar water splitting, Sol. Energy. 155 (2017) 770-780; A. A. M. Ibrahim, I. Khan, N. Iqbal, A. Qurashi, Facile synthesis of tungsten oxide—Bismuth vanadate nanoflakes as photoanode material for solar water splitting, Int. J. Hydrogen Energy. 42 (2017) 3423-3430; and I. Khan, A. Qurashi, Shape Controlled Synthesis of Copper Vanadate Platelet Nanostructures, Their Optical Band Edges, and Solar-Driven Water Splitting Properties, Sci. Rep. 7 (2017) 14370, each incorporated herein by reference in their entirety]. Pseudo flower shaped BV microparticles having a current density of 0.7 mAcmwere synthesized using ultrasonication approach assisted by hydrothermal treatment. The BV microparticles were obtained with good yield within a short reaction time using this approach because ultrasonication could improve separation of the BV microparticles. In addition, it was found that temperature played a role in controlling the morphology and overall PEC performance of the resulting BV microparticles [I. Khan, S. Ali, M. Mansha, A. Qurashi, Sonochemical assisted hydrothermal synthesis of pseudo-flower shaped Bismuth vanadate (BiVO) and their solar-driven water splitting application, Ultrason. Sonochem. 36 (2017) 386-392, incorporated herein by reference in its entirety].

According to a first aspect, the present disclosure relates to a method of preparing bismuth vanadate particles. The method involves the steps of (i) mixing a bismuth(III) salt and an acidic aqueous solution to form a bismuth mixture, (ii) mixing a metavanadate salt and an basic aqueous solution to form a vanadate mixture, (iii) mixing the vanadate mixture and the bismuth mixture to form a reaction mixture that is simultaneously subjected to ultrasonication, and (iv) hydrothermally treating the reaction mixture, thereby forming the bismuth vanadate particles.

A particle is defined as a small object that behaves as a whole unit with respect to its transport and properties. An average particle size (e.g., average diameter) of the particle, as used herein, and unless otherwise specifically noted, refers to the average linear distance measured from one point on the particle through the center of the particle to a point directly across from it. For polygonal shapes, the term “particle size”, as used herein, and unless otherwise specified, refers to the greatest possible distance measured from a vertex of a polygon through the center of the face to the vertex on the opposite side. For a circle, an oval, an ellipse, and a multilobe, “particle size” refers to the greatest possible distance measured from one point on the shape through the center of the shape to a point directly across from it.

Morphologically, BiVO(BV) exists in four different polymorphs, including pucherite, clinobisvanite, dreyerite, and scheelite-type tetragonal phases [R. L. Frost, D. A. Henry, M. L. Weier, W. Martens, Raman spectroscopy of three polymorphs of BiVO4: clinobisvanite, dreyerite and pucherite, with comparisons to (VO)-bearing minerals: namibite, pottsite and schumacherite, J. Raman Spectrosc. 37 (2006) 722-732; and S. Tokunaga, H. Kato, A. Kudo, Selective preparation of monoclinic and tetragonal BiVOwith scheelite structure and their photocatalytic properties, Chem. Mater. 13 (2001) 4624-4628, each incorporated herein by reference in their entirety]. Among these, the tetragonal phase is considered highly photoactive and has been explored as a photocatalytic material [I. Khan, S. Ali, M. Mansha, A. Qurashi, Sonochemical assisted hydrothermal synthesis of pseudo-flower shaped Bismuth vanadate (BiVO) and their solar-driven water splitting application, Ultrason. Sonochem. 36 (2017) 386-392, incorporated herein by reference in its entirety]. The morphology of BiVOmay have a strong impact on the photocatalytic activity. Research efforts are needed to synthesize BiVOwith controlled morphologies and faceted crystallinities in order to further enhance the PEC performance. Synthetic protocols with proper reaction parameters can lead to an effective morphology design. By controlling various reaction parameters, various synthetic techniques have been used to design specific morphologies (e.g., BiVOnanowires [S. Kim, Y. Yu, S. Y. Jeong, M. G. Lee, H. W. Jeong, Y. M. Kwon, J. M. Baik, H. Park, H. W. Jang, S. Lee, Plasmonic gold nanoparticle-decorated BiVO/ZnO nanowire heterostructure photoanodes for efficient water oxidation, Catal. Sci. Technol. 8 (2018) 3759-3766; J. Resasco, H. Zhang, N. Kornienko, N. Becknell, H. Lee, J. Guo, A. L. Briseno, P. Yang, TiO/BiVOnanowire heterostructure photoanodes based on type II band alignment, ACS Cent. Sci. 2 (2016) 80-88; and N. Iqbal, I. Khan, Z. H. A. Yamani, A. Qurashi, A facile one-step strategy for in-situ fabrication of WO—BiVOnanoarrays for solar-driven photoelectrochemical water splitting applications, Sol. Energy. 144 (2017) 604-611, each incorporated herein by reference in their entirety], BiVOnanocones [Y. Qiu, W. Liu, W. Chen, G. Zhou, P. C. Hsu, R. Zhang, Z. Liang, S. Fan, Y. Zhang, Y. Cui, Efficient solar-driven water splitting by nanocone BiVO-perovskite tandem cells, Sci. Adv. 2 (2016), incorporated herein by reference in its entirety], spherical BiVO[D. Channei, A. Nakaruk, W. Khanitchaidecha, P. Jannoey, S. Phanichphant, Hybrid high-porosity rice straw infused with BiVOnanoparticles for efficient 2-chlorophenol degradation, Int. J. Appl. Ceram. Technol. 16 (2019) 1060-1068, incorporated herein by reference in its entirety], and flower-shaped BiVO[K. Zhang, J. Deng, Y. Liu, S. Xie, H. Dai, Photocatalytic Removal of Organics over BiVO—Based Photocatalysts, in: Semicond. Photocatal. —Mater. Mech. Appl., InTech, 2016; H. Cai, L. Cheng, F. Xu, H. Wang, W. Xu, F. Li, Fabrication of the heterojunction catalyst BiVO/P25 and its visible-light photocatalytic activities, R. Soc. Open Sci. 5 (2018); and S. Dong, J. Feng, Y. Li, L. Hu, M. Liu, Y. Wang, Y. Pi, J. Sun, J. Sun, Shape-controlled synthesis of BiVOhierarchical structures with unique natural-sunlight-driven photocatalytic activity, Appl. Catal. B Environ. 152-153 (2014) 413-424, each incorporated herein by reference in their entirety).

The bismuth vanadate particles of the present disclosure may be in the form of particles of the same shape or different shapes, and of the same size or different sizes. The bismuth vanadate particles may have a polyhedron shape, such as a rectangular shape, a prismatic shape (e.g., a hexagonal prism, a triangular prism), and a pyramidal shape (e.g., a triangular-based pyramid, a square-based pyramid), and other polyhedron shapes. Alternatively, the bismuth vanadate particles have a combination of planar sides with rounded edges or corners.

A regular polyhedron, also termed “Platonic solid”, is defined herein as a polyhedron composed of regular polygons having equal face angles and equal edge lengths, where the regular polygons meeting identically at every vertex. As used herein, a regular octahedron is a Platonic polyhedron composed of eight equilateral triangles with four of which meeting at every vertex. A non-regular polyhedron (e.g., non-regular octahedron) is defined herein as a polyhedron composed of non-regular polygonal faces. Non-regular polygons have at least two different face angles, and may have equal or unequal edges. In some embodiments, the bismuth vanadate particles have a Platonic shape, such as regular tetrahedral, cubic, hexagonal, octahedral, dodecahedron, and icosahedron shapes. In preferred embodiments, the bismuth vanadate particles have an octahedral shape, more preferably a regular octahedral shape. Alternatively, the bismuth vanadate particles may have a non-regular octahedral shape.

Surface roughness is a component of surface texture. It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth. In one embodiment, the bismuth vanadate particles of the present disclosure are in the form of regular or non-regular octahedrons having a microscopically smooth surface (see). Alternatively, the bismuth vanadate particles may have an uneven/rough surface (see,E-F). The uneven surface may have an irregular contour that is cracked, bumpy, jagged, spiky, serrated, or zigzag.

The bismuth vanadate particles of the present disclosure may comprise BiVOin the form of an amorphous phase, a crystalline phase, or both. Preferably, the bismuth vanadate particles comprise or consist essentially of crystalline BiVO. The amount of crystalline BiVOmay be measured by X-ray diffraction patterns. In one embodiment, the bismuth vanadate particles consist essentially of crystalline BiVO, meaning that the bismuth vanadate particles comprise at least 99 wt %, preferably 99.9 wt %, more preferably 99.95 wt % BiVOin a crystalline state, relative to a total weight of the particles.

The bismuth vanadate particles may comprise crystalline BiVOhaving monoclinic (clinobisvanite BiVO, scheelite BiVO) crystal symmetry, tetragonal (dreyerite BiVO) crystal symmetry, orthorhombic (pucherite BiVO) crystal symmetry, or a mixture thereof. Preferably, the bismuth vanadate particles comprise or consist essentially of crystalline BiVOof tetragonal crystal symmetry. In one embodiment, the bismuth vanadate particles consist essentially of crystalline BiVOhaving tetragonal crystal symmetry, meaning that the bismuth vanadate particles comprise at least 90 wt %, preferably 95 wt %, more preferably at least 99 wt % crystalline BiVOrelative to a total weight of the bismuth vanadate particles, and this crystalline BiVOhas tetragonal crystal symmetry. In some embodiments, where the bismuth vanadate particles comprise less than 100 wt % crystalline BiVOhaving a tetragonal crystal symmetry relative to a total weight of the bismuth vanadate particles, the BiVOthat is not crystalline BiVOhaving a tetragonal crystal symmetry may be amorphous BiVO, or BiVOhaving a different crystal symmetry (e.g., monoclinic, orthorhombic).

In one or more embodiments, the bismuth vanadate particles of the present disclosure have an average particle size in a range of 0.8-6 μm, preferably 1-5.5 μm, preferably 1.5-5 μm, preferably 2-4.5 μm, preferably 2.5-4 μm, preferably 3-3.5 μm. However, in certain embodiments, the average particle size of the bismuth vanadate particles is less than 0.8 μm or greater than 6 μm.

The bismuth vanadate particles may be agglomerated or non-agglomerated (i.e., the bismuth vanadate particles are well separated from one another and do not form clusters). As used herein, the term “agglomerates” refers to a clustered particulate composition comprising primary particles, the primary particles being aggregated together in such a way so as to form clusters thereof, at least 50 volume percent of the clusters having a mean diameter that is at least 2 times the mean diameter of the primary particles, and preferably at least 90 volume percent of the clusters having a mean diameter that is at least 5 times the mean diameter of the primary particles. The primary particles may be the bismuth vanadate particles having an average particle size as those previously described. In one embodiment, the bismuth vanadate particles are agglomerated and the agglomerates have an average diameter in a range of 2-50 μm, 4-25 μm, or 8-10 μm. Preferably, the bismuth vanadate particles of the present disclosure are non-agglomerated.

Dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. In probability theory and statistics, the coefficient of variation (CV), also known as relative standard deviation (RSD) is a standardized measure of dispersion of a probability distribution. It is expressed as a percentage and is defined as the ratio of the standard deviation (σ) of to the mean (μ, or its absolute value |μ|). The CV or RSD is widely used to express precision and repeatability. It shows the extent of variability in relation to the mean of a population. The bismuth vanadate particles having a narrow size dispersion, i.e., monodispersity, is preferred. As used herein, “monodisperse”, “monodispersed” and/or “monodispersity” refers to particles having a CV or RSD of less than 25%, preferably less than 20%, preferably less than 10%, preferably less than 8%, preferably less than 6%, preferably less than 5%. The bismuth vanadate particles may be monodisperse with a coefficient of variation or relative standard deviation (ratio of the particle size standard deviation to the particle size mean) of less than 25%, less than 20%, less than 15%, less than 12%, less than 10%, less than 9%, less than 8%, less than 7%, less than 6%, less than 5%, or preferably less than 2%.

The method of preparing the bismuth vanadate particles involves mixing a bismuth salt and an acidic aqueous solution to form a bismuth mixture. In preferred embodiments, the bismuth salt is a bismuth(III) salt. Exemplary bismuth salts include, but are not limited to, bismuth(III) nitrate, bismuth(III) nitrate pentahydrate, bismuth(III) sulfate, bismuth(III) acetate, bismuth(III) chloride, bismuth(III) bromide, bismuth(III) iodide, bismuth(III) phosphate, bismuth hydroxide, bismuth(III) citrate, bismuth(III) oxynitrate, bismuth(III) oxychloride, and the like. In one embodiment, more than one type of bismuth(III) salt may be used. Preferably, the bismuth(III) salt is bismuth(III) nitrate, bismuth(III) nitrate pentahydrate, or a mixture thereof. The bismuth salt may be present in an amount of 10-5,000 mM, preferably 25-2,500 mM, preferably 50-1,000 mM, preferably 75-500 mM, preferably 100-250 mM relative to a total volume of the bismuth mixture.

The acidic aqueous solution comprises an acid. The acid employed herein may be an inorganic acid or an organic acid, and specifically includes, but is not limited to, nitric acid, hydrochloric acid, sulfuric acid, phosphoric acid, sulfamic acid, fluoroboric acid, hexafluorophosphoric acid acetic acid, fluoroacetic acid, chloroacetic acid, dichloroacetic acid, trifluoroacetic acid, trichloroacetic acid, propionic acid, formic acid, citric acid, citrate acid, oxalate acid, and benzoic acid. Preferably, the acid is nitric acid. In one embodiment, the acid employed herein has a molarity of 0.5-8 M, preferably 1-6 M, more preferably 2-5 M, or about 4 M. In a further embodiment, the pH of the bismuth mixture may be in a range of from about 0 to 5, from about 0.1 to 4, from about 0.4 to 3, from about 0.6 to 2, or from about 0.8 to 1. Most preferably, the pH of the bismuth mixture is about 0.

The bismuth mixture may further comprise an ionic surfactant, preferably an anionic surfactant. Exemplary anionic surfactants include, but are not limited to, alkylbenzene sulfonates (e.g., sodium dodecylbenzene sulfonate (SDBS), sodium octylbenzene sulfonate, sodium propylbenzene sulfonate), alkyl sulfonates (e.g., sodium octane sulfonate, sodium decane sulfonate, sodium dodecane sulfonate), and alkyl sulfate (e.g., sodium lauryl sulfate (sodium dodecyl sulfate), lithium dodecyl sulfate, sodium octyl sulfate). In preferred embodiments, the ionic surfactant employed herein is sodium dodecylbenzene sulfonate. The ionic surfactant may be present in an amount of 5-4,000 mM, preferably 15-2,000 mM, preferably 30-800 mM, preferably 50-400 mM, preferably 75-200 mM relative to a total volume of the bismuth mixture. In one or more embodiments, a molar ratio of the bismuth(III) salt to the ionic surfactant present in the bismuth mixture is in the range of 1:2 to 3:1, preferably 1:1 to 2:1, preferably 5:4 to 3:2, or about 4:3.

The method disclosed herein also involves mixing a metavanadate salt and a basic aqueous solution to form a vanadate mixture. Exemplary metavanadate salts that may be suitable for the method described herein include, but are not limited to, ammonium metavanadate, sodium metavanadate, potassium metavanadate, silver metavanadate, cesium metavanadate, and the like. In one embodiment, more than one type of metavanadate salt may be used. Preferably, the metavanadate salt is ammonium metavanadate. The metavanadate salt may be present in an amount of 10-5,000 mM, preferably 25-2,500 mM, preferably 50-1,000 mM, preferably 75-500 mM, preferably 100-250 mM relative to a total volume of the vanadate mixture. In one or more embodiment, a molar ratio of the bismuth(III) salt present in the bismuth mixture to the metavanadate salt present in the vanadate mixture is in the range of 1:2 to 2:1, preferably 2:3 to 3:2, preferably 4:5 to 5:4, or about 1:1.

The basic aqueous solution comprises a base. The base employed herein may be a hydroxide base, such as an alkali metal hydroxide (e.g., lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, and cesium hydroxide), an alkali earth metal hydroxide (e.g., magnesium hydroxide, calcium hydroxide, strontium hydroxide, and barium hydroxide), and an ammonium hydroxide (e.g., ammonium hydroxide, tetramethylammonium hydroxide, triethylammonium hydroxide, trimethylanilinium hydroxide, etc.). Preferably, the base is sodium hydroxide. In one embodiment, the base employed herein has a molarity of 0.25-4 M, preferably 0.5-3 M, more preferably 1-2.5 M, or about 2 M. In a further embodiment, the pH of the vanadate mixture may be in a range of 9-14, 10-13.8, 11-13.5, 12-13.2, or 12.5-13. Most preferably, the pH of the vanadate mixture is about 14.

The acidic aqueous solution and the basic aqueous solution may each comprise water as a solvent. The water may be tap water, distilled water, bidistilled water, deionized water, deionized distilled water, reverse osmosis water, and/or some other water. In one embodiment, the water is bidistilled to eliminate trace metals. Preferably the water is bidistilled, deionized, deionized distilled, or reverse osmosis water and at 25° C. has a conductivity at less than 10 μS·cm, preferably less than 1 μS·cm, a resistivity greater than 0.1 MΩ·cm, preferably greater than 1 MΩ·cm, more preferably greater than 10 MΩ·cm, a total solid concentration less than 5 mg/kg, preferably less than 1 mg/kg, and a total organic carbon concentration less than 1000 μg/L, preferably less than 200 μg/L, more preferably less than 50 μg/L. In an alternative embodiment, other solvents, such as alcohols (e.g., methanol, ethanol, i-propanol, n-butanol) may be used in addition to, or in lieu of the water to form the bismuth mixture and the vanadate mixture. The mixing steps described above may occur by stirring, shaking, sonicating, blending, or by otherwise agitating the mixture.